1. Field of the Invention
This invention relates to overcurrent trip units for circuit breakers, and more particularly to such trip units providing protection for the neutral conductor which is adjustable separately of the line protection functions.
2. Background Information
In three-phase electric power systems servicing normal, balanced loads, no current, or minimal current, flows in the neutral conductor. However, in installations where single phase loads draw current from a three-phase supply substantial currents can be generated in the neutral conductor which can cause overheating. Such neutral conductor overheating can be caused by inexpensive power supplies used in some computers. Such loads can generate a third harmonic on the neutral conductor having an amplitude which is the square root of 3 times the line current. In new installations, the neutral conductor can be sized to accommodate such neutral currents. However, in many older installations in which the wiring is no longer easily accessible, the neutral conductor may typically be one-half the size of the phase conductors.
In the United States, codes do not permit opening the neutral conductor However, interruption of current in all of the phase conductors terminates neutral current. While separable contacts are not provided in the neutral conductor, some circuit breakers monitor neutral current and trip upon detection of selected neutral overcurrent conditions. Typically, the sensed neutral current is auctioneered along with the phase currents with desired scaling of neutral current relative to the phase currents being fixed by selection of the per unit value of the current transformer sensing neutral current relative to the per unit values of the current transformers sensing phase current. Such fixed scaling is set at the factory and is not easily changed.
There is a need for a circuit breaker trip unit with neutral protection which is operator adjustable separate from the phase protection. This would allow a single model of a circuit breaker to be easily adapted in the field to be used, for instance, with an installation in which the neutral conductor is one/half the size of the phase conductors, or an installation in which the neutral conductor is twice the size of the phase conductor. Preferably, this separately adjustable neutral protection would include long delay trips, short delay trips and instantaneous trips.